1. Field of the Invention
The present invention relates to a method and a device for uniformly and slightly dissolving a surface of a semiconductor substrate or a thin-film surface layer, such as a polysilicon film, formed on a semiconductor substrate, (hereinafter collectively referred to as "a semiconductor substrate").
The present invention is effective in analyzing the impurities contained in the surface of a semiconductor substrate and in cleaning the surface of a semiconductor substrate by removing the impurities.
2. Description of the Related Art
If metallic impurities, such as Na, K, and Fe, are present in the semiconductor substrate, even in a small quantities, they have adverse effects on electrical characteristics of semiconductor elements, as is known in the art.
As a semiconductor device is developed toward a highly-dense and highly-integrated one, the presence of the impurities extremely near the surface of the semiconductor substrate becomes a significant problem.
To improve the electrical characteristics of semiconductor elements, it is necessary to prevent contamination with impurities or to remove impurities from the surface of the semiconductor substrate. To achieve this, it is important to know accurate amounts of the impurities contained in the semiconductor substrate and to determine a vertical distribution profile of the impurities near the surface of the semiconductor substrate.
Hitherto, to dissolve the surface layer of the semiconductor substrate, a device shown in FIGS. 3 and 4 has been used. In such a conventionally-used device, a semiconductor substrate 1 is mounted on a fluoroplastic holding stage 2. On the semiconductor substrate 1, a fluoroplastic frame 3 is mounted and fixed watertight by means of screws at the four corners, thereby forming a hole surrounded by the frame 3. Into the hole, a dissolving solution A, which is a mixed solution 5 of hydrofluoric acid and nitric acid, is supplied in a volume needed for dissolving a predetermined amount (a predetermined depth) of the surface layer of the semiconductor substrate. Subsequently, the surface layer is allowed to stand for a predetermined time period while being exposed to the dissolving solution. In this manner, a predetermined region in the surface of the semiconductor substrate is dissolved selectively to a predetermined depth.
The solution dissolving impurities thus obtained is subjected to a flame-less atomic absorption device to measure the amount of the impurities.
In the dissolving solution consisting of nitric acid, which is responsible for oxidization, and hydrofluoric acid, when the concentration of hydrofluoric acid is high, a dissolution rate of the surface layer is controlled by nitric acid. In contrast, when the concentration of nitric acid is high, hydrofluoric acid determines the dissolution rate.
To be more specific, when nitric acid is diluted with acetic acid or water, nitric acid is activated. Therefore, the oxidation reaction of nitric acid controls the dissolution rate. If no diluent is added, the dissolution is controlled only by diffusion of hydrofluoric acid.
In short, in the case where the concentration of nitric acid is high, the dissolution rate of the surface of the semiconductor substrate is preferentially regulated by the diffusion of hydrofluoric acid contained in a low amount. The crystal orientation and electrical conductivity of the semiconductor substrate do not influence the dissolution rate of the surface of the semiconductor substrate. Hence, it is important to stir the dissolving solution.
However, there are the following problems with the conventional methods.
First, since the dissolving solution is directly supplied to the surface of the semiconductor substrate present at the bottom of the hole surrounded by the frame in a volume enough to dissolve a predetermined amount (predetermined depth) of the semiconductor substrate, the semiconductor substrate exposed first to the dissolving solution initiates the reaction with the dissolving solution, immediately upon exposure, as indicated by curve a shown in FIG. 2. The portion first exposed to the dissolving solution is therefore dissolved much (dissolved deeper) than the portion finally exposed to the dissolving solution. As a result, the surface of the wafer is dissolved not uniformly in depth, forming a rough surface.
Second, since the dissolving solution is supplied to the hole in as a low volume as possible, approximately 3 seconds is required to spread the dissolving solution over the entire surface of a semiconductor substrate if a 6-inch semiconductor substrate is used. As a result, if the dissolving solution is supplied at the same flow rate, longer time is required for spreading the solution over the entire surface of the semiconductor substrate as the diameter of the substrate increases.
Third, if a diluent, such as acetic acid, is added to the dissolving solution, the dissolving solution is contaminated with impurities ascribed to the diluent, with the result that the impurity concentration of the background is undesirably increased.
Fourth, when nitric acid and hydrofluoric acid is mixed uniformly and sufficiently with stirring manually or mechanically, there is a problem in that the dissolving solution is easily splashed out of the hole, even if the dissolving solution is used in as a low volume as possible.
In addition, when a stirrer is used as mechanical stirring means, the dissolving solution is possibly contaminated with impurities due to the stirrer. Consequently, the background is contaminated with impurities.
The background impurity contamination occurs also when the entire surface of the semiconductor substrate is dissolved, since the dissolving solution is kept by means of a holding tool consisting of a frame, holding stage and screws. Accordingly, the contamination ascribed to the holding tool is caused since it is exposed to the dissolving solution all the time.